You do have to have iTunes running for the AppleTV to stream and/or sync your media.

I'm not really sure what a MediaSmart server is, but if it can be mounted as a drive in OS X, then you just need to go into iTunes, go to prefs/advanced, and make sure to uncheck the option for having iTunes copy the music.

Then you would just need to drag the media into iTunes. It will be added to the iTunes library, but will not be copied to the iMac.

That being said, the AppleTV will only play mp4/h.264 video files that meet certain specifications. So, if your media is not in that format then all of the above is a moot point.